Typically, coated glass articles are produced by continuously coating a glass substrate while it is being manufactured in a process known in the art as the "Float Glass Process". This process involves casting glass onto a molten tin bath which is suitably enclosed, then transferring the glass, after it has sufficiently cooled, to lift-out rolls which are aligned with the bath, and finally cooling the glass as it is advanced across the rolls initially through a lehr and thereafter while exposed to the ambient atmosphere. A non-oxidizing atmosphere is maintained in the float portion of the process, while the glass is in contact with the molten tin bath to prevent oxidation. An air atmosphere is maintained in the lehr. The chemical vapor deposition of various coatings may be conveniently performed in the bath or the lehr, or even in the transition zone therebetween.
The physical form of the reactants employed in glass coating processes is generally a gas, liquid, solid, vaporized liquid or solid, liquid or solid dispersed in a carrier gas mixture, or vaporized liquid or solid dispersed in a carrier gas mixture. The chemical vapor deposition process generally employs a vaporized liquid or solid, which is typically dispersed in a carrier gas mixture.
Chemical vapor deposition processes are well known in the art of coating glass substrates. U.S. Pat. No. 4,100,330 discloses a process for coating a glass substrate with a first layer of silicon and a second layer of a metal oxide deposited by the pyrolytic decomposition of a metal coating compound vapor at the surface of the hot substrate in an oxidizing atmosphere.
U.S. Pat. No. 4,847,157 discloses a process for coating a glass substrate with a first silicon-containing layer, a titanium nitride-containing layer overlaying the first layer, a second silicon-containing layer covering the titanium nitride-containing layer, and an optional abrasion resistant layer, e.g., comprising tin oxide, on the second silicon-containing layer.
U.S. Pat. No. 4,692,180 discloses a method for spraying a powdered metal compound directly onto the surface of a hot glass ribbon produced by the float glass process, wherein the powder pyrolytically decomposes to prepare a metal oxide coating U.S. Pat. No. 3,852,098 discloses the vaporization of dispersed powdered metal compounds by a hot carrier gas, which is then directed onto the surface of a hot glass substrate to deposit a metal oxide coating. A similar patent employing solid metal compounds is U.S. Pat. No. 2,780,553, wherein a fixed bed of metal coating compound is vaporized by contact with a hot carrier gas. Finally, U.S. Pat. No. 4,351,861 discloses a process for fluidizing a heated to vaporize the suspended particles, and directed onto the surface of a hot glass substrate to form a coating. These methods, employing solid coating precursor particles produce reactant streams which are subject to concentration variations due to fluctuations in particle sizes, changes in particle surface area over time, difficulties in conveying solid materials at a steady rate, etc.
The prior art also includes processes whereby organometallic salts are solubilized in acid or a hydrocarbon, and thereafter vaporized in a hot carrier gas. U.S. Pat. No. 4,571,350 discloses a process for spraying an atomized mist of a metal salt solution into a fuming chamber. The solution is vaporized and thereafter delivered to the surface of a hot glass substrate. U.S. Pat. No. 3,970,037 discloses dissolving a coating reactant into a solvent, which is then sprayed into a hot carrier gas where it is vaporized and then directed onto the surface of a hot glass substrate. In both cases, the reactant pyrolitically decomposes to produce an oxide coating, but the solubilizing agents interfere with the molecular transport at the surface of the glass, thereby causing variations in the deposition.
Yet another method for producing thermally decomposable metal vapor streams for chemical vapor deposition processes comprises bubbling a hot carrier gas through a metal salt in liquid form, such as is disclosed in U.S. Pat. Nos. 4,212,663 and 4,261,722. U.S. Pat. No. 3,808,035 discloses passing an inert gas sweep through a bubbler to produce a gas stream having a low precursor concentration, and thereafter directing the gas stream into contact with a substrate at a temperature of 100.degree. C. to 300.degree. C. Although the bubbling process provides a method for vaporizing liquid coating precursors directly into a carrier gas it suffers from several disadvantages which diminish its usefulness for preparing vaporized reactants for chemical vapor deposition. Primarily, the bath of liquid coating precursor must be maintained at a temperature near its vaporization temperature, during the entire vaporization process, in order to insure a high concentration of vaporized reactant in the carrier gas. This elevated bath temperature, maintained over an extended period of time, can accelerate decomposition of the coating precursors, some of which are very heat sensitive. In addition, the specific heat of vaporization required to vaporize the liquid causes the temperature of the bath to decrease as the carrier gas is bubbled through the compound. The decreasing bath temperature, which is difficult to remedy in a uniform manner using outside heat sources, causes the vapor pressure for the liquid to decrease, thereby causing a steadily decreasing concentration of vaporized precursor in the carrier gas stream. Finally, in a bubbling process where the liquid bath contains two or more coating precursors, each having a different pure component vapor pressure, the more volatile component will vaporize preferentially, thereby changing the partial vapor pressure of the liquid components, and consequently changing the concentrations of the vaporized reactants in the carrier gas stream as the liquid bath is depleted.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.
It would be desirable to be able to vaporize coating precursors or mixtures thereof such that a uniform, steady stream of concentrated coating precursor vapor is produced, which would allow the formation of thicker deposited layers than those obtainable by the prior art processes, while at the same time providing greater control for the deposition of the coating.